1. Field of the Invention
The present invention relates to generally an endoscope, and more specifically an imaging system for use in an endoscope, which system includes a solid-state imaging device picking up an image formed by a vari-focal lens.
2. Description of the Prior Art
It is well known that an endoscope is very useful for observing the inside of a cavity or opening of a living body or mechanical equipment. Proposed was an endoscope which employs a solid-state imaging device, such as a charge coupled device (CCD), and a bucket brigade device (BBD), generally known as a charge transfer device (CTD), rather than a so-called image guide consisting of a bundle of optical fibers.
In general, an endoscope employs conventionally a lens system having a relatively wide-angle viewing field so as to be larger in depth of focus, thus eliminating focus adjustment operations. An endoscope is however required that is equipped with a vari-focal lens such as a zoom lens. Such an endoscope generally includes a lens system composed of several groups of lenses, between which the spacings are adjustable to establish zooming effects. With an endoscope having an image bundle conveying optical image signals formed by a zoom lens, it is necessary to dispose the lens in such a position that the optical axis of the lens is substantially perpendicular to an imaging or focal plane of the fiber bundle. This requires that the position of the exit pupil of the zoom lens be far from the imaging end surface of the image bundle.
A zoom lens system for use conventionally in a still camera, a movie camera, or a television camera usually includes a focusing lens, and performs by itself variable magnification functions together with mechanical compensation functions for movement of the focal plane resultant from the former functions. Due to compactness in configuration of endoscopes, however, it is not advantageous for a zoom lens system for use in an endoscope to be equipped with the compensation functions by means of mechanical structure. In the case of an endoscope emplying an image bundle of optical fibers, it is also not advantageous to design its imaging system in such a manner that the image bundle is movable in dependence upon the operation of changing magnification of the zoom lens for compensation for the movement of the focus of the lens because of the lengthy structure of the bundle.